A multitude of pump constructions are known. For expository convenience, the present invention is illustrated with reference to a small (i.e. less than 200 G.P.H.) gear pump. It will be recognized, however, that the principles of the invention are equally applicable to a variety of different pump and other systems.
Gear pumps are conventionally housed in sealed enclosures from which the pump drive shaft extends. A drive motor is then coupled to the shaft to drive the pump head.
In such "direct drive" systems, dynamic bearing seals are required on the drive shaft to prevent fluid leaks. This seal presents a number of problems. Primary among these is the difficulty of maintaining a leak-free seal without imposing excessive frictional loading, which decreases pump efficiency and increases heat losses. Related problems include decreased reliability, decreased pump lifetime, and contamination due to seal failures.
To overcome these problems, U.S. Pat. Nos. 3,238,883 and 4,111,614 teach the use of a magnet drive system wherein a drive shaft needn't extend through the pump housing. Instead, as shown in FIGS. 1 and 2, the drive shaft 10 has mounted thereon a magnet 12 that is enclosed within a fluid-tight cylindrical extension 14 of the pump housing 16. Outside this portion of the housing is disposed a cylindrical driving magnet 18 that is magnetically coupled to the magnet inside the housing. By rotating the external cylindrical magnet 18, the magnet 12 on the drive shaft inside the housing, and thus the drive shaft 10, are caused to rotate.
While the foregoing magnet drive system eliminates the need for a dynamic shaft bearing seal, it still requires a motor 20 to drive the outside magnet. Motors typically include a pair of motor bearings 22 (FIG. 2) in which the motor shaft 24 is journalled for rotation. Such bearings are prone to failure, reducing the reliability of the associated pump. The bearings also introduce friction, which decreases efficiency and increases heat losses. These problems are common to any system employing a drive motor, including the direct drive systems discussed above.
A second problem common to both direct and magnet drive systems is the use of brushed motors. In such motors, electrically conductive "brushes" are spring biased against the rotor shaft to make electrical connections with windings mounted thereon. Due to their reliance on an electromechanical contact against a moving element, motor brushes are prone to intermittent contacts and ultimately failure.
A third problem common to both the direct and magnet drive systems is the space that the motor requires. Pump assemblies are often the largest components in the apparatuses in which they are used. Of this size, the pump drive motor generally contributes more than half. If the size of the composite pump head/motor/controller assembly could be reduced, significant economies of space, and consequently of money, could be achieved in the apparatuses in which pumps are employed.
From the foregoing it will be recognized that there is a need for a compact pump assembly that operates without motor shaft bearings or shaft seals.
In accordance with the present invention, the foregoing need is fulfilled by integrally incorporating a motor as part of the pump assembly. By so doing, the motor shaft bearings and seals can be eliminated, and a compact pump assembly can be provided.
In more detail, a pump according to the present invention includes a rotary drive shaft which, at one end, is connected to the pump head and, at the other end, is connected to a permanent magnet rotor. This shaft and rotor are enclosed in a cavity defined by a fluid-tight cup that has an open first end and a closed second end. The cavity in which the rotor rotates is in fluid connection with the pump head through its open first end. Outside the cup enclosure are disposed a plurality of electrical stator windings. A housing encloses all of the foregoing elements, and may further enclose a circuit board that includes controller/driver circuitry for controlling operation of the motor and driving the stator windings. The circuit board also desirably includes circuitry for sensing the speed at which the motor is operating, and for adjusting the drive currents applied to the stator windings in response thereto. This feedback provides a "stiff" motor, highly immune to changing load conditions.
By the foregoing construction, a highly compact, efficient, and reliable pump is provided.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description thereof, which proceeds with reference to the accompanying drawings.